Applicants have directed their attention to making miscible blends of polymers having utility as composite matrices for advanced composite aerospace uses and for use as general purpose injection molding resins. Rather than directing attention to miscible thermoplastic blends, the prior art has been directed to blends of thermoplastics without regard to whether or not they are miscible.
U.S. Pat. No. 4,293,670 to Robeson et al describes molding compositions of blends of poly(aryl ether) resin and a polyetherimide resin. The poly(aryl ether) resin is taught to be a linear, thermoplastic polyarylene polyether wherein the arylene units are interspersed with ether, sulfone or ketone linkages. Particular formulae of polyetherimides are taught in the reference. A molding composition comprising blends of these materials is claimed and exemplified in the reference.
U.S. Pat. No. 4,713,426 describes blends of a biphenyl containing poly(aryl ether sulfone) and a poly(aryl ether ketone). The reference teaches that these blends have limited miscibility and excellent mechanical compatibility. According to the reference, these blends possess, in an article molded therefrom, a good balance of properties including higher modulus, impact resistance, solvent resistance and resistance to environmental stress cracking. As taught, the poly(aryl ether ketones) are used because they offer an exceptional balance of properties, namely, high melting point, excellent thermal stability, excellent hydrolytic stability, high stiffness and strength, good toughness, and excellent solvent and environmental stress rupture resistance. The reference specifically teaches that the results indicate that a low level of miscibility is observed in these blends as the glass transition temperature of the poly(aryl ether sulfone) is slightly decreased. However, the blend contains separate glass transition temperatures for each of the components of the blends.
U.S. Pat. No. 4,684,674 to Brooks teaches another blend. Specifically, the reference teaches polyamide-imidephthalamide copolymers and polyamide-imide copolymers containing aromatic sulfone polymers. However, the blends are two-phase and exemplified by two glass transition temperatures, one for each of the components of the blend. The reference teaches that a completely miscible system exhibits a single glass transition temperature (T.sub.g) while immiscible blends have two T.sub.g 's, each at the temperature of one of the components. The reference specifically teaches that the polyamideimide/polyethersulfone blends taught in the reference show no evidence of a system which is miscible.
World Patent Application No. WO 8604-079A describes blends of phenylindane containing polyimides with polyetherimides, polysulfones, polyarylether ketones, polycarbonates, polyarylates or polyphenylene oxides. The compositions are claimed to be useful as adhesives, coatings or matrix resins in carbon fiber reinforced composites.
In the field of miscibility or compatibility of polymer blends, the prior art has found predictability to be unattainable, even though considerable work on the matter has been done. According to the authorities:
(A) "It is well known that compatible polymer blends are rare". Wang and Cooper, Journal of Polymer Science, Polymer Physics Edition, Vol. 21, p. 11 (1983).
(B) "Miscibility in polymer--polymer blends is a subject of widespread theoretical as well as practical interest currently. In the past decade or so, the number of blend systems that are known to be miscible has increased considerably. Moreover, a number of systems have been found that exhibit upper and lower critical solution temperatures, i.e., complete miscibility only in limited temperature ranges. Modern thermodynamic theories have had limited success to date in predicting miscibility behavior in detail. These limitations have spawned a degree of pessimism regarding the likelihood that any practical theory can be developed that can accommodate the real complexities that nature has bestowed on polymer-polymer interactions." Kambour, Bendler, Bopp, Macromolecules, 16, 753 (1983).
(C) "The vast majority of polymer pairs form two-phase blends after mixing as can be surmised from the small entropy of mixing for very large molecules. These blends are generally characterized by opacity, distinct thermal transitions, and poor mechanical properties. However, special precautions in the preparation of two-phase blends can yield composites with superior mechanical properties. These materials play a major role in the polymer industry, in several instances commanding a larger market than either of the pure components." Olabisi, Robeson, and Shaw, Polymer--Polymer Miscibility, Academic Press, New York, N.Y., p. 7 (1979).
(D) "It is well known that, regarding the mixing of thermoplastic polymers, incompatibility is the rule and miscibility and even partial miscibility is the exception. Since most thermoplastic polymers are immiscible in other thermoplastic polymers, the discovery of a homogeneous mixture or partially miscible mixture of two or more thermoplastic polymers, is, indeed, inherently unpredictable with any degree of certainty; for example, see P. J. Flory, Principles of Polymer Chemistry, Cornell University Press, 1953, Chapter 13, p. 555." Younes, U.S. Pat. No. 4,371,672.
(E) "The study of polymer blends has assumed an ever increasing importance in recent years and the resulting research effort has led to the discovery of a number of miscible polymer combinations. Complete miscibility is an unusual property in binary polymer mixtures which normally tend to form phase-separated systems. Much of the work has been of a qualitative nature, however, and variables such as molecular weight and conditions of blend preparation have often been overlooked. The criteria for establishing miscibility are also varied and may not always all be applicable to particular systems." Saeki, Cowie and McEwen, Polymer, vol. 25, p. 60 (January 1983).
Thus, miscible or compatible polymer blends are not common. The criteria for determining whether or not two polymers are miscible are now well established. According to Olabisi, et al., Polymer--Polymer Miscibility, supra p. 120:
"The most commonly used method for establishing miscibility in polymer--polymer blends or partial phase mixing in such blends is through determination of the glass transition (or transitions) in the blend versus those of the unblended constituents. A miscible polymer blend will exhibit a single glass transition between the Tg's of the components with a sharpness of the transition similar to that of the components. In cases of borderline miscibility, broadening of the transition will occur. With cases of limited miscibility, two separate transitions between those of the constituents may result, depicting a component 1-rich phase and a component 2-rich phase. In cases where strong specific interactions occur, the Tg may go through a maximum as a function of the concentration. The basic limitation of the utility of glass transition determinations in ascertaining polymer--polymer miscibility exists with blends composed of components which have equal or similar (20.degree. C. difference) Tg's, whereby resolution by the techniques to be discussed of two Tg's is not possible." PA0 "Perhaps the most unambiguous criterion of polymer compatibility is the detection of a single glass transition whose temperature is intermediate between those corresponding to the two component polymers."
W. J. MacKnight et al., in Polymer Blends, D. R. Paul and S. Newman, p. 188, Academic press, New York, N.Y. (1978) state:
In this passage, it is clear that by compatibility the authors mean miscibility, i.e., single phase behavior. See, for example, the discussion in Chapter 1 by D. R. Paul in the same work. The above references and related application are hereby incorporated by reference.
However, the miscible blends disclosed in this application are one phase based on them having a single glass transition temperature giving the resulting blends improved chemical resistance compared to the immiscible multiphase blends having similar constituents. Further, the miscible blends show improved thermal and optical performance over immiscible blends. This is attributable to discovering that select imide containing polymers having an isopropylidene group are miscible with biphenyl containing polysulfones. Herein, the term "polyimide" refers to polymers containing the following linkage: ##STR1## or mer unit.